Notion Relations is the way to have in your Notion workspace to link two different databases that are related in some conceptual or logical way. It allows you to establish some grouping between parent and child databases or use it as you can use foreign keys in a standard relational database for those reading this that have a technical background.

How Do you define a link between two databases?

That is the first question we would like to answer, and as already commented, Notion relations is the concept that helps us to do this job. To create a notion relation between two databases, you need to create a property in one of these databases that are going to be of the particular type of Relation, as shown in the picture below:

Now you have already done this, and you need to select what is the new database that you want to link as part of the notion relation definition, as you can see in the sample in the picture below:

All the different databases will be available, and you will have some filter search to simplify the location of your target database when you have a large amount of them as part of your Notion workspace.

How Many Different Notion Relation Types Exist?

The main thing you need to be aware of when you are defining a Notion relation between two databases is which of these components should have the attribute, and this will depend on the use-case that you are implementing, as the different types commented in below list:

• Equal-To-Equal Relationship: That means that you have a relationship from one to one between both databases. For example, that means that a single notion database item will be related to exactly one thing on the target database item, and the relationship is “equivalent,” so there is no parent-child relationship between them. Some samples of this can be between a Customer and an Invoice.
• One-To-One Relationship: That means that the relationship is similar to the previous one, but there is a parent-child relationship between them, and in this case, the recommendation is the same to define one databases but allow to show in the other notion database to allow navigation that is bi-directional between them as shown in the picture below:

• One-To-Many Relationship: In this scenario, the parent database item will have many of the notion database items. The primary sample can be meeting notes assigned to a customer or activities as part of a travel plan. In this case, you will need to define the property only on the child notion database that will allow the navigation and the grouping from the parent, as shown in the picture below:

• Many-To-Many Relationship: This scenario means that one notion database item can be linked with many other notion database items. At the same time, each notion database item can also be linked to many of the notion database items in the original one. It is the most complex relationship, and some samples can be Meeting and Participants if you want to track at the same time who is attending each of the specific meetings and at the same time you want to see in what meeting a specific participant has been attended
  

How Do a Group Elements in the Notion Relations?

To show the notion of database items that depend on a specific parent item as part of the Notion Relation, you need to select the target database you want to add to the page and then add a Filter to that Notion Database to perform the job as shown in the picture below:

Notion Relation Best-Practices

You can also use the notion of relation combined with the different database views and pages to create a different perspective of that relationship. For example, you can use the same Customer Meeting relationship to show the Customer meetings you have on the customer page and, at the same time, on the Meeting page to be able to filter or sort it for customers.

So, once you define the Notion relation, you need to think about all the use-cases you can fulfill with the same defined Notion relation because probably, for each of these Notion relations, you want to see more than one perspective or view that will help you to improve your efficiency on the task you are performing at the moment. You can always see more information on the official documentation here